In conventional freezing of food with CO.sub.2, liquid CO.sub.2 is obtained from a storage tank at about 0.degree. F. and about 300 psig and is injected into a freezer. As the pressurized liquid CO.sub.2 flashes down to atmospheric pressure, it forms a mixture of solid CO.sub.2 ("dry ice") and cold CO.sub.2 vapor. The sublimation of the solid CO.sub.2 and the warming of the vapor CO.sub.2 from the sublimation temperature (-109.3.degree. F.) to the desired freezer temperature (usually between -100.degree. F. and -50.degree. F.) provide the refrigeration needed to freeze the food. The food to be frozen enters and exits the freezer continuously, and is moved through the freezer on a conveyor. After warming to the freezer temperature, the CO.sub.2 vapor leaves the freezer through the entrance and exit openings, and sometimes also through an exhaust line.
Food freezing typically requires about a one to one (1:1) ratio by weight of liquid CO.sub.2 to food. As a result, large amounts of CO.sub.2 vapor are lost to the atmosphere by conventional freezing. For example, a single standard spiral freezer unit can consume 10,000 tons of CO.sub.2 per year. The cost of CO.sub.2 represents most of the freezing cost. Recovery and recycling of CO.sub.2 from the freezer could substantially lower the cost of freezing food. Furthermore, recycling could produce environmental benefits, such as reduced CO.sub.2 emissions to the atmosphere and reduced truck transportation of liquid CO.sub.2.
Unfortunately, processes used in conventional merchant plants for liquefaction and purification of CO.sub.2 from gas streams would be uneconomical for freezer recycle. Merchant liquid CO.sub.2 is produced from industrial byproduct sources containing typically 98% CO.sub.2 (all CO.sub.2 concentrations are given in mole percentage, dry basis) and higher. In contrast, typical food freezers operate at CO.sub.2 concentrations of 40-80%, due to air infiltration through the freezer openings. The CO.sub.2 vapor exhaust is typically further diluted with room air to warm the exhaust and prevent ice build up in the exhaust system. Vent losses of CO.sub.2 from conventional liquefaction and purification processes increase greatly as the concentration of CO.sub.2 in the feed gas declines. These losses make the cost of conventional processes too high for economical use in recovering CO.sub.2 from typical freezers.
Freezers may be modified to reduce air infiltration and to supply higher CO.sub.2 concentrations to a CO.sub.2 recycle system. However, even at 90% CO.sub.2 concentration in the freezer, conventional CO.sub.2 liquefaction and purification methods would suffer losses of about 29%, which are still too large to be economical.
A simple, economical, and reliable process is needed for the liquefaction and purification of CO.sub.2 from vapor recovered during food freezing. To be economical, the process must have substantially lower losses of CO.sub.2 than conventional methods.
A conventional process for liquid CO.sub.2 production is shown in FIG. 1. A raw gas source 1 is obtained as an industrial byproduct, such as from an ammonia plant, petroleum refinery, or fermentation source. Raw gas characteristics vary, but for economical production using conventional methods, the CO.sub.2 concentration is typically above 95%. Raw gas sources are typically obtained at ambient temperature or above, and are often saturated with water. For exemplary purposes, the raw gas source 1 in FIG. 1, is assumed to contain 98% CO.sub.2 at a pressure of 20 psia and a temperature of 90.degree. F. The gas passes through heat exchanger 2 and is chilled to 50.degree. F. against ammonia refrigerant. As a result, water condenses out of the gas and is removed in separator 4.
Cooled gas stream 5 is compressed by compressor 6 to 75 psia, and the heat of compression is removed in exchanger 8. Gas stream 9 leaving exchanger 8 is cooled to 95.degree. F., which allows separation of condensed water in separator 10. Gas stream 11 is compressed by compressor 12 to about 315 psia (stream 13), cooled in heat exchanger 14 to 50.degree. F., and fed as stream 15 to separator 16 where condensed water is separated. Cooled gas stream 17 is treated by adsorbent driers 18 to remove further water to achieve a low dew point, typically about -80.degree. F. The driers operate in swing mode, with one bed in operation while the other bed is being regenerated. Dry gas stream 19 is used to provide heat to the reboiler of CO.sub.2 purification column 20. Dry gas stream 21 leaving the reboiler is now near the dew point for CO.sub.2 condensation. The dry gas 21 is partially condensed at -5.degree. F. against ammonia refrigerant in exchanger 22, providing a two-phase feed stream 23 for column 20.
Purification column 20 is a distillation column of well-known design to those skilled in the art. Typically, column 20 is a packed bed column, though other types of distillation column designs may be used. Liquid CO.sub.2 is increasingly purified as it flows down the column, and exits at the column bottom as liquid stream 27 at high purity, typically at about 99.9% CO.sub.2. Vapor stream 24 exits at the top of the column and contains all of the non-condensable portions of the raw gas feed, gases such as methane, hydrogen, nitrogen, etc. Some of the CO.sub.2 content of this vent stream is condensed against ammonia refrigerant in exchanger 25 and the CO.sub.2 liquid produced flows back into column 20 as reflux. The final vent stream 26 is released to the atmosphere.
Liquid CO.sub.2 stream 27 is often subcooled to about -20.degree. F., 305 psia, in exchanger 28 against ammonia refrigerant. This subcooling allows use of lower pressure storage vessels at production plants and minimizes vaporization of the liquid as it is pumped. After delivery to a customer site, the liquid CO.sub.2 is stored as a saturated liquid at about 295-305 psig (310-320 psia), and 0.8-2.7.degree. F. For simplicity, storage conditions at the customer site are characterized as 300 psig, 0.degree. F. herein.
The varied characteristics of raw gas sources require many variations of the conventional process described above. Some of the separation stages for removal of water may not be needed, or the pressures and temperatures may differ from the example. Also, additional unit operations are often needed for removal of contaminants such as hydrocarbons or sulfur compounds. The refrigerant used is usually ammonia or cooling water, or a combination thereof, depending on the temperature level in a particular exchanger. In addition, other refrigerants have been used, such as hydrocarbons and chlorofluorocarbons.
Not shown in FIG. 1 is a companion utility process of ammonia refrigeration, typically supplying liquid ammonia at different pressure levels to provide refrigeration at temperatures between -25.degree. F. and 100.degree. F. The design of this process is well known in the art, and is widely used in CO.sub.2 production and in many other industrial and commercial processes.
Conventional methods of CO.sub.2 liquefaction, using ammonia refrigeration at -25.degree. F., suffer increasing CO.sub.2 losses as the purity of the raw gas decreases. A typical conventional system can maintain a vent condenser temperature as low as -20.degree. F., assuming ammonia refrigeration at 1 psig, -25.degree. F. suction, and a 5.degree. F. approach in the condenser. The -20.degree. F. vent condenser temperature produces a vent composition of 73% CO.sub.2. For a typical merchant plant feed of 98% CO.sub.2, this would limit losses to 5.5% of the feed CO.sub.2 content, as shown by the -20.degree. F. temperature line in FIG. 2. However, if the feed were vapor recovered from a freezer at 90% CO.sub.2, the conventional process would suffer vent losses of 30%, making the process uneconomical. Hence, these losses make conventional processes uneconomical for recycle of recovered vapor from food freezing with CO.sub.2.
Losses of CO.sub.2 in the vent condenser may be reduced by increasing column pressure or by reducing the condensing temperature. As cited in U.S. Pat. No. 4,952,223 to Kirshnamurthy et al., increasing the pressure has disadvantages, including increased power consumption, decreased product quality, and the potential for formation of an azeotrope between CO.sub.2 and oxygen. Higher pressure would also increase the cost of the equipment. Decreasing the vent condenser temperature is a better solution.
FIG. 3 presents the vent loss (percent of the feed stream CO.sub.2 content lost in the column vent stream) as a function of the vent condenser temperature, for the case of a 90% CO.sub.2 feed stream (typical of vapor recovered from a freezer that is used as a feed for the invention disclosed below). As shown in FIG. 3, the vent condenser loss for a 90% CO.sub.2 feed stream is 30% at a conventional condenser temperature of -20.degree. F., but only 8.3% at a reduced temperature of -50.degree. F. Loss rates of up to 10% should not prevent the system from achieving economical operation, and indeed are typical of many merchant CO.sub.2 plants. The reduced loss rate achieved by a -50.degree. F. vent condenser temperature is also shown in FIG. 2 for other feed concentrations.
To achieve vent condenser temperatures below -20.degree. F. with conventional designs requires vacuum operation in the ammonia refrigeration system. Vacuum operation is undesirable, causing problems such as leakage of air into the ammonia system, larger vapor line sizes, higher power requirements, and lubrication problems. These problems increase equipment and operating costs and decrease reliability. Other refrigerants, such as chlorofluorocarbons, may be used instead of ammonia to achieve reduced vent temperatures. However, refrigeration systems employing these alternative refrigerants are more expensive, may have operational problems when employed for CO.sub.2 liquefaction, and have environmental concerns.
Conventional CO.sub.2 liquefaction methods have been applied, though without economic success, to recycle CO.sub.2 in food freezing. Duron et al., "Reliquefies CO.sub.2 For Cryogenic Freezing, Food Engineering", April 1972, p. 72-74, described a system developed for recovery, liquefaction, purification, and recycle of CO.sub.2 in food freezing. In the described system, CO.sub.2 gas was obtained from the freezer through ducts on the entrance and exit of a spiral freezer. A blower was used to transfer the gas to the recycle system. In the recycle system, the vapor was compressed, cooled, dried, treated for odor removal, condensed, and purified before returning to the freezer. Two parallel six-stage centrifugal compressors were used, with intercooling on each stage. The system used three mole sieve beds for drying and odor removal, and a four stage Freon refrigeration system. This recycle system was reported to cost $1 million in 1972.
There were several disadvantages to the Duron et al. system. The dual six-stage compressors and four-level refrigerant system were complex and expensive. The centrifugal compressors were limited in turndown and required anti-surge flow control. The estimated power requirement was very high, when compared with conventional CO.sub.2 plant power requirements of less than 200 kWh/ton. There is no indication that means were used to reduce losses of CO.sub.2 in the purification system.
U.S. Pat. No. 4,952,223 to Kirshnamurthy, et al., describes a process to recycle CO.sub.2 from food freezing using pressure swing adsorption (PSA). The process may be used for feed streams containing about 35% to 98% CO.sub.2. When used to recover CO.sub.2 from food freezers, however, the expected feed CO.sub.2 concentration is significantly less than 89% by volume and may be as low as 35% by volume. The patent states that commercial refrigeration units employing liquid CO.sub.2 for freezing foods contaminate the liquid CO.sub.2 with nitrogen and oxygen (air) to the extent that the spent CO.sub.2 vapor may contain as much as 50% or more of the contaminants. In this process, vapor drawn from the freezer is sent down a recovery line by injection of warm, pressurized air. The vapor is compressed, cooled, dried, condensed, and purified by conventional techniques. The purification column vent is treated with a PSA unit. In the PSA unit, the CO.sub.2 from the column vent is adsorbed onto a solid adsorbent. The CO.sub.2 is recovered from the solid adsorbent, at low pressure, and the low pressure CO.sub.2 stream from the PSA is recompressed with a vacuum pump and sent to the suction of the feed compressor. The PSA unit is used to reduce the very high vent loss that a conventional process would experience with such a low concentration feed gas.
Since the vapor from the freezer is further diluted with air before recovery, refrigeration content is wasted, and the injected air increases compression and purification costs. The system is complex, requiring multiple adsorbent beds operated in swing cycles, vacuum pumps, and extra compression to recover the low-pressure CO.sub.2. These factors increase the cost and reduce the operating reliability of the system.
U.S. Pat. No. 5,186,008 to Appolonia et al., discloses a method to increase the CO.sub.2 concentration in the vapor removed from a freezer for recovery, thus addressing one of the disadvantages of U.S. Pat. No. 4,952,223, above. Exhaust plenums are used on the entrance and exit of a spiral freezer, with the draw rate varied with injection rate of cryogen, to reduce air infiltration. A second blower is used to draw vapor for recovery from the freezer bottom, where the CO.sub.2 concentration is substantially higher than at the top because of density differences. The mass flow of recovery vapor is controlled to be equal to the mass flow of injected cryogen times a constant (90% is given). This is done to avoid over or under pressure in the freezer, which would expel more than a minimum amount of CO.sub.2, or would introduce too much air. The recovered vapor is to be recycled by the PSA process of U.S. Pat. No. 4,952,223.
Selective membranes may be used to reduce the loss of CO.sub.2 in the purification column vent. U.S. Pat. No. 4,639,257 to Duckett et al. proposes treating the column vent from a conventional CO.sub.2 process with a selective membrane. The vent is first heated, and then allowed to pass through a membrane unit where the CO.sub.2 selectively permeates the membrane. The high purity, low pressure CO.sub.2 permeate is then sent to the suction of the feed compressor for recovery. For low concentration feeds, a second membrane unit is proposed to increase the concentration of the feed prior to liquefaction and purification. U.S. Pat. No. 4,990,168 to Sauer et al. proposes a similar process, except that the vent stream is not heated prior to the membrane unit.
Both membrane processes suffer disadvantages. High pressures (400-415 psia) are required to feed gas to the membrane units, and the CO.sub.2 permeate is obtained at low pressure (22 psia) requiring substantial recompression. Also, membrane units are expensive and may be subject to fouling by process contaminants.
While U.S. Pat. No. 4,990,168 teaches that it is not necessary to heat the vent stream going to the membrane, and higher selectivities are achieved, the low temperatures reduce the permeability of the membrane and increase the required surface area and expense.
U.S. Pat. No. 4,977,745 to Heichberger describes a process for production of liquid CO.sub.2 from low purity sources using turbine expansion of the non-condensable impurities to provide refrigeration. The feed gas is cooled to remove water, compressed, dried, and partially condensed. The vapor from the condensation step, containing the impurities, is heated and expended in a multistage turbine expander. The cold turbine exhaust is used for refrigeration in the process, such as for the CO.sub.2 condenser.
The Heichberger process is proposed for raw gas feeds containing less than 85% CO.sub.2, particularly flue gas containing less than 50% CO.sub.2. Feed gas with more than 85% CO.sub.2 may not contain enough impurities to provide sufficient refrigeration upon expansion. While it is possible to operate food freezers at less than 50% CO.sub.2 concentration, such low CO.sub.2 concentrations in the freezer indicate air infiltration, which reduces freezer efficiency. Also, the capacity of freezer recovery units would be small compared to merchant CO.sub.2 production plants, and the cost of turbine expansion units taught in the Heichberger patent are high for such small plants. Furthermore, Brayton cycles as used in the Heichberger patent required much more energy than the Rankin cycles used in conventional merchant CO.sub.2 liquefaction. These factors tend to make the Heichberger process uneconomical for CO.sub.2 recovery from freezers.